1. Field of the Invention
The present invention relates to an electron gun to be employed in a cathode ray tube.
2. Description of the Prior Art
In an electron gun of the type employed in a cathode ray tube, an electron beam or electron beams are emitted from a single or plurality of cathodes and the single electron beam or a plurality of electron beams are converged by a single principal lens. In general, such an electron gun is divided into two regions of an object point forming region (a cathod lens region) and a principal converging region (a principal lens region). The characteristics of the electron beam on the fluorescent screen are determined by these two lens regions. Since a circular beam spot is to be produced by ordinary electron guns, the above mentioned object point forming region and principal converging region are frequently formed of electrodes arranged in a rotatory symmetric configuration.
The deflecting magnetic field to be provided by the electromagnetic deflecting means in the cathode ray tube is a so-called convergence free deflecting magnetic filed (hereinafter to be called a CFD magnetic field) of which the horizontal deflecting magnetic filed (1) forms a pin magnetic field and the vertical magnetic filed (2) forms a barrel magnetic field as shown in FIG. 19. Hence, in an electron gun with the electrodes arranged in a rotatory symmetric configurations as above, the focal distance (hereinafter the f value) of the electron beam is the same in both the vertical direction and the horizontal direction, i.e., the vertical f value (f.sub.V) and the horizontal f value (f.sub.H) are equal (f.sub.V =f.sub.H). The electron beam (4) when influenced by the CFD magnetic field is, particularly at the corners of the screen (3), overfocused in the vertical direction (refer to FIG. 21) and underfocused in the horizontal direction (refer to FIG. 20). This is due to the lensing action or characteristics of the CFD magnetic field, that is, when an electron beam is deflected to the corner portion, a convex lensing action is developed in the vertical direction and concave lensing action is developed in the horizontal direction. Therefore, when seen on the screen (3), as shown in FIG. 22, a circular beam spot (4a) is obtained at the center of the screen and optimal focusing is effected there, but an elongated beam spot (4b) is formed at the corners and is accompanied by a halo (5).
Under these circumstances, it has been known that the quality of the picture image on a color cathode ray tube, particulary the beam spots, becomes deteriorated at the peripheral portion of the screen and, in particular, at the four corners. On the other hand, there are trends for flattening the screen and consumerization of intermediateresolution cathode ray tubes, which have resulted in increasing demands for improvements of the beam spot quality at the corners. For example, in a character display, improvement of beam spots at the screen corners is strongly desired.
There has been developed a system to obtain an optimally focused beam spot with less halo at the corners by providing a weaker convex lens in the vertical direction and a stronger convex lens in the horizontal direction. In other words, the horizontal focal distance (f.sub.H) is less than vertical focal distance (f.sub.V) (i.e., f.sub.H &lt;f.sub.V). In this case, however, a circular beam spot cannot be obtained at the center of the screen (3) because, when the horizontal focusing was adjusted to be optimal, the vertical focusing became underfocused, and the obtained beam spot (4a) at the center became a vertically elongated one as shown in FIG. 23.
These and other objects of this invention will become apparent from the following disclosure and appended claims.